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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Sep 2020, 7:54 am   #21
Craig Sawyers
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Default Re: KT66 gm?

Apart from lots of stupidity, there are several 0.1uF metal shell capacitors in 600V rating here https://www.hificollective.co.uk/ from about a fiver each. Not sure of the dimensions needed though.

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Old 24th Sep 2020, 8:48 am   #22
GrimJosef
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Default Re: KT66 gm?

Quote:
Originally Posted by Radio Wrangler View Post
... Little subtleties like this tend to get lost as time goes by.

It can be overcome by adding a pair of small capacitors, but first you need to know of the need.... then you need to know the value. It might be helpful if someone with an original amplifier in working condition measured it ...
That's such a good idea David that Keith Snook did it quite a few years ago now - reported in some detail, with examples of possible replacements here https://www.keith-snook.info/quad-ii...amplifier.html. I take my hat off to him.

If further support is needed for this, I see that when Quad re-released the amp as the Quad II Classic they fitted polyprop capacitors in the C2 and C3 positions, each with a 22pF disc ceramic from the output wire to ground.

Cheers,

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Old 24th Sep 2020, 10:20 am   #23
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Default Re: KT66 gm?

Thanks GJ, for posting Keith Snook's diagram. He clearly expects the KT66's to draw their full wollop of 65mA. My knowledge of Quads is just about nil, but I can see & understand that the amp specialists here have concerns about imbalance.
How old is peterp1's Amp I wonder ? Still no info yet on the "electronics man's" Valve Tester or proper Ia & mA/V results at the Book's value for Vg ?

Regards, David
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Old 24th Sep 2020, 10:58 am   #24
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Default Re: KT66 gm?

Oh, well, no prize for me, but I'm glad it's been done.

Kudos to Keith Snook for spotting a bit of jiggery pokery and demystifying solutions. relying on unspecified strays of components never looks wonderful when you're found out. Things like this are a lot foxier in the RF world than in the audio world.

Capacitance direct to ground at these points will tend to promote RF oscillation in the KT66, but that doesn't seem to be a common problem with Quad IIs though I've seen mention of people fitting G1 stoppers and other mentions of care needed with screen decoupler choice including adding smaller high frequency types.

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Old 24th Sep 2020, 11:39 am   #25
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Default Re: KT66 gm?

There isn't an explicit KT66 screen decoupler in the Quad II. The 'shifting round' of the common element of the output transformer windings from the screen circuit to the cathode means that the screens are held to AC ground just by the 16uF HT smoothing cap. Although the wiring goes round the houses a bit you're right, there really doesn't seem to be a problem with output stage oscillation in this amp.

I'm not sure how critical this stray capacitance is. I've seen plenty of Quad IIs with C2 and C3 replaced by polyprops and they seem to work fine. In that sense I suspect Peter Walker wasn't really 'relying' on it. I suppose it's interesting though that he chose a Hunts Mouldseal for C1 but used the, presumably more expensive, metal-canned type L45 for C2 and C3.

I never met him but I understand he could be, shall we say, 'robust' if challenged technically. I suspect he'd have argued a) that his 1950's/60's customers weren't likely to start swapping components in and out, and if they did and doing so bit them then they only had themselves to blame, and b) given the probably more serious impact of all sorts of strays in the output transformer he wasn't going to feel too guilty if the capacitor he'd explicitly selected had a stray that turned out to help a bit .

Cheers,

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Old 24th Sep 2020, 4:17 pm   #26
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Default Re: KT66 gm?

Just a thought, not having heard from peterp1 - - who was his local "electronics man" with a valve tester? I recall, two or three months or so back, there was a flurry of Forum thread posts from someone down in Cornwall who had knackered a CT160 & a VCM MK3. Cant remember if it was Launceston, or elsewhere. (Testing KT66's perhaps !).

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Old 24th Sep 2020, 11:36 pm   #27
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Default Re: KT66 gm?

Quote:
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Please peterp1, any chance on some info on the circuitry in which your KT66's are being used? A Circuit Diagram would be great.
Also, perhaps you could ask your local electronics man about which Valve Tester he used, & obtain info on the Anode Currents (Ia) achieved, Anode & Screen Voltages used, and the Grid Voltages used ? Or the Valve Data Book he used ?
For example, if he used the AVO Valve Data Manual(which quite a few of us use), Va = 250V, Vs = 250V, Vg = -15V, and expected Anode Current (Ia) is 65mA. Gm = mA/V & should be near enough 6.3 mA/V. Assuming the valve is in good condition. Some Valve Testers(or Tube Testers) just give a coloured meter display. I.e. RED zone = Fail, WHITE zone(middle of the meter) = maybe just usable, and GREEN zone = Pass.

Regards, David
That's roughly what I was going to say, the gm is normally quoted at the chosen point on the valves characteristic curve.
e.g. AVO use an Anode Current of 65mA for the KT66 and 57mA for the KT88.
For the KT66 they chose -15V to give that 65mA and expect a gm of 6.3.
The meter on an AVO tester will have a Red Fail zone for a gm of 4 or less.
The Green Pass zone is for a gm of 5.6 to 10.4.
So another tester type may choose a different Anode Current point on the graphs and the gm may be subsequently different.
I use my valve tester to do a plot with various grid voltages and I consider it to be a most excellent way of matching valves in addition to verifying their health Les-G4CNH
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Old 25th Sep 2020, 12:25 am   #28
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Default Re: KT66 gm?

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I never met him but I understand he could be, shall we say, 'robust' if challenged technically. I suspect he'd have argued a) that his 1950's/60's customers weren't likely to start swapping components in and out
Now there's a thought. Could any of the leading lights of the fifties and sixties, Peter Walker, Harold Leak, Arthur Radford, etc. have ever imagined what the hifi industry would turn into by our current era?

They could have imagined us having flying cars, conversational robots, holidays on Mars, but tube rollers and directional cables? Not a chance!

I think they'd have something very direct to say.

David
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Old 25th Sep 2020, 1:17 am   #29
Matthew kane
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Default Re: KT66 gm?

I always thought AVO's 65mA expected anode current was a typo and if you go by the datasheet and other parametric tester valve books, 85mA should be right around the ballpark for the expected plate current reading based off the 250/250v plate/screen voltage and -15v for grid bias.
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Old 25th Sep 2020, 6:44 am   #30
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Default Re: KT66 gm?

Quote:
Originally Posted by Radio Wrangler View Post
Now there's a thought. Could any of the leading lights of the fifties and sixties, Peter Walker, Harold Leak, Arthur Radford, etc. have ever imagined what the hifi industry would turn into by our current era?

They could have imagined us having flying cars, conversational robots, holidays on Mars, but tube rollers and directional cables? Not a chance!

I think they'd have something very direct to say.
I understand that Peter Walker did have something to say about the cable fetish. It was to the effect that it was very important to have the “right” cables. For example, if they were too short to reach the speakers, they would have a major deleterious effect on the sound quality….


Cheers,
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Old 25th Sep 2020, 9:28 am   #31
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Default Re: KT66 gm?

And of course the famous (possibly apocryphal) story of the Quad engineer being asked at a demo what was the special orange cable being used to connect the speakers - "Lawnmower cable" was the answer.
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Old 25th Sep 2020, 6:23 pm   #32
David Simpson
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Default Re: KT66 gm?

Lacking a response or further info from "peterp1", those of us who are valve testing &/or audio amp enthusiasts are floundering in the surf. We are tossing, hither & thither, lifebelts - without knowing exactly where the troubled swimmer is.
I.e. is this a new acquisition by Peter, or perhaps a previous owner has done some mods ?
Or perhaps his local "electronics man" has been delving inside ?
Here speaks an ex RAF S & R guy, who had collegues down at St Mawgan. We were used to helping guys & damsels in distress.

Regards, David
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Old 25th Sep 2020, 7:18 pm   #33
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Default Re: KT66 gm?

I guess there's not a lot more can be said without some more information to go on. Would be nice to get some feedback or an outcome, though.

David
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Old 26th Sep 2020, 7:41 pm   #34
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Default Re: KT66 gm?

Thank you gentlemen for your kind responses.
I do not know the make or model of his valve tester. He has marked some of the valves thus:


5.9/43
4.3/37
6.3/53
5.5/41
5.7/41

They were from Quad 2 amplifiers. Is there any harm in using them? They sound ok in use.

Regards,
Peter, Launceston, Cornwall.
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Old 26th Sep 2020, 7:47 pm   #35
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Default Re: KT66 gm?

That differs from what you said in post #1
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Old 26th Sep 2020, 8:03 pm   #36
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Default Re: KT66 gm?

I'm confused..J.
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Old 26th Sep 2020, 8:09 pm   #37
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Default Re: KT66 gm?

I'm assuming the first figure is mA/V and the second anode current in mA.
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Old 26th Sep 2020, 8:21 pm   #38
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Default Re: KT66 gm?

Perhaps the intended meaning was something along the lines, "Some of the KT66s I've had tested show low gm compared to the listed specification, only around 3/4 of the figure for a "good" valve". That would apply to e.g the one labelled "4.3/37" if the 4.3 means the gm of that particular valve with 6.3 being a good KT66 figure. (OK, not exactly 3/4 the value, but maybe there are a few low ones in the region of 3/4 "good"). I'm further guessing that the second figure is anode current in mA at a particular bias setting.

"Harm" is unlikely to ensue, but pairing up valves with similar characteristics is good practice- in the case of the Quad II, giving similar standing currents precedence over gm figures, so that a valve with a somewhat higher current at a given bias doesn't tend to hog current over the other- a characteristic of the common cathode resistor set-up. The 4.3/37 specimen would seem to be in its twilight years compared to the others.
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Old 26th Sep 2020, 8:46 pm   #39
David Simpson
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Default Re: KT66 gm?

Me thinks that Peter aught to research Push-Pull Amp circuitry &/or get someone to coach or mentor him.
Non of the valves are anywhere up to scratch, except for the one with 6.3mA/V & an Ia of 53mA. For O/P configuration it needs to be paired with another KT66 of very close Ia & Gm. The two 41mA ones might possibly do for drivers in another type of amp.
Peter, are you just using your amp, or amps to blast out Status Quo, or for your own guitar playing ?

Regards, David
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Old 27th Sep 2020, 3:33 pm   #40
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Default Re: KT66 gm?

Quote:
Originally Posted by peterp1 View Post
... They were from Quad 2 amplifiers. Is there any harm in using them? They sound ok in use.
KT66s can be prone to going gassy, especially if they have been out of use for a long time. A gassy valve is a risky option in a Quad II as the grid leak resistance is on the high side and the common cathode resistor won't control a faulty valve's current as effectively as two separate resistors would. Gas can remain adsorbed onto the valve's internal metalwork until everything gets properly hot. Most valve testers never get a KT66 properly hot, so unless the gas level is very high indeed they can miss it.

Amp damage resulting from a gassy valve usually makes itself very clear, but unfortunately not until it's too late to do much about it. If you're running successfully with the valves now, and have been for extended periods, then they may be OK. Far and away the best test for gas is to monitor the valve's DC grid voltage with respect to ground. It should be less than 0.5V and stay that way when the amp gets properly hot.

Cheers,

GJ
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